Electronic angle limiting device for servo mechanism system

ABSTRACT

Angle limiting is achieved in a tangent servo system by modifying the sine input to the servo system. The product of the cosine input and the appropriate one of two constants representing the tangents of the maximum excursion angles is selected by control circuits and switched into the servo motor drive circuit when such maximum excursion is reached. An unmodified sine input is provided by the control circuits for normal operation between maximum excursion limits.

United States Patent Day et a], i

[54] ELECTRONIC ANGLE LIMITING DEVICE FOR SERVO MECHANISM SYSTEM Inventors: Richard A. Day, Playa Del Rey; William E. French; Michael A. Riley, both of Los Angeles, all of Calif.

Assignee: The United States of America as represented by the Secretary of the Air Force Filed: June 15, 1971 Appl. No.: 153,311

us. c1. ..3l8/626, 318/66! rm. Cl. ..G05g 5/00 Field of Search ..3l8/626, 627, 661, 608

ins] 3,662,244

[ 51 May 9,1972

[56] References Cited UNITED STATES PATENTS 3,403,310 9/1968 Davidolf ..3l8/626 3,525,9l8 8/1970 Parnell ..3l8/626X Primary vaminer-Benjamin Dobeck Atlorney-Harry A. Herbert, Jr., et al.

[57] ABSTRACT unmodified sine input is provided by the control circuits for normal operation between maximum excursion limits.

1 Claim, 4 Drawing Figures v5 Limrnz ELECTRONIC ANGLE LIMITINGDEVICE FOR SERVO MECHANISM SYSTEM BACKGROIJND OF THE INVENTION This invention relates to tangent servo systemsthat use direction cosines to provide angular positioning information,

and in particular to improved electronic angle limiting means .necessary to modify the servo amplifier to accommodate the gain switching. Limitswitches further have accuracy limitations and require frequent adjustment and appreciable maintenance. There currently exists, therefore, the need for servo system angle limiting devices that are not subject to these limitations. The present invention is directedtoward providing such improved devices.

SUMMARY OF THE INVENTION In servo systems of the type comprehended by the present invention the servo motor is driven by a supply voltage e, which may be represented as:

e,=jcos-isin6 (1) wherein j and i are vector quantities and 0 is the position angle. The present inventionutilizes the ratio of j to i to determine the position of the servo. Angle limits are set by making j equal to 1' tan 0 (0 being the desired limit angle). This is accomplished by switching circuits and control circuits that substitute the i input as modified by. a constant (resistance .arrangement that produces the desired tan 0) for the j input when the limit angle has been reached or exceeded.

It is a principal object of the invention to provide a newand improved electronic angle limiting device for a servo mechanism system.

It is another object of the invention to provide an angle limiting device for a servo mechanism system that .does not require modification of the servo package.

It is another object of the invention to provide an angle limiting device for a servo mechanism system that does not require modification of the servo amplifier.

These, together with other objects, features and advantages of the invention, will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically a servo mechanism system of the type comprehended by the invention;

FIG. 2 is a vector diagram of a two-dimensional pointing problem;

FIG. 3 is a block diagram of one presently preferred embodiment of the invention; and

FIG. 4 is a schematic diagram of a device incorporating the principles of the invention to achieve angle limiting at two points.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The electronic angle limiter of the present invention will be described having reference to the simple follow-up resolver system of FIG. I. In this system the device 9 to be positioned is mechanically rotated by servo motor 6 through mechanical linkage 8. Angle excursion is limited by limiter 10 as hereinafter described.

. conventions:

e ,=jcos0isin0 2 since e, 0 at null isin0=jeos0 (2a) and j/i==sin0/cos0=tan0. 3

In FIG. 1, i and j represent position vectors of a two-dimensional pointing problem. The vector geometry is shown in FIG. 2

In the amplifier 7 motor 6 combination of FIG. 1, the motor 6 will driveuntil e, is reduced to 0. From standard resolver The salient point of the development is that at null j/i is equal mally for j to the tangent of the angle to which the system positions. Referring to Equation (2) the following can be stated:

. i= Kcos 0 j Ksin 0 at. null. The electronic angle limiter of the present invention is based on the fact that the ratio'of j to 1 determines the position of the servo, not-the absolute value of either term. Thus by making the ratio constant it is possible to achieve a constant angular position irrespective of absolute values of either i or j. By modifying the j or sine input to the servo, it is possible to achieve this constant ratio for any desired angular displacement greater than or less than some constant. This is easily demonstrated starting with Equation (2a). It is desired to modify j such that the servo positions to some angle 0 and limits at this angle even if j should increase further. When the servo limits thefollowing equation will obtain:

' j/i=sin 0/cos 0=tan 4 where tan :1 is a constant. Thus by setting j t tan (1 the position will be constant at for j sin 4: and will position norsin 4:. In practice the electronic angle limiter sets two limits with a band of normal operation between the A block diagram of the limiting circuit is shown in FIG. 3.

In operation phase sensitive demodulator l2 and detector 13 in combination with the constant tan d5, (designated by block 1 5) detects when the angular excursion reaches a first limit and consequently closes switch S1 and and opensswitch S1"S2. This removes j from the circuit and replaces it with i tan 4n. When-the-second excursion limit is reached the control circuit comprising phase sensitive demodulator l1 and detector 12 in combination with tan (block 16) closes switch S2 and opens switch S]. This replaces 1' tan 11:, with i tan Between excursion limits the drive circuit operates normally with switch Sl*S2 being closed and switches S1 and 82 being open.

By way of example, consider the case of limiting at +2 and 1 2. Let tan 4;, tan l 2 and tan tan 2. The circuit has three possible states:

0 +2 First examine the case of 0 ---1 2. The inputs to the SI detector arejanditan 12. For 0 l2, lj/ itan 12/. The detector senses this and turns on the FET with S] on the gate. The sin 0 output is set to 1 tan l2 and the servo positions to l 2. For 1 2 '0 2 the two detectors are off and the FET labeled S1 "S2-is on and the system operation is normal.

A circuit which uses this principle is illustrated in FIG. 4. Normal operation j drive is provided when field efiect transistor switch 33 is closed. A first limit drive circuit consists of resistors 37, 38,, amplifier l7 and field effect transistor switch 32. This represents the tan (2) limit drive circuit. The tan (l2) limit drive circuit comprises resistors 43, 44 and field efl'ect transistor switch 34. Resistor 52 represents the load of the succeeding stage. The control circuit for the 2 limit comprises resistors 39, 40, a phase sensitive democulator consisting of field effect transistor 20 and its modulator reference source, and the detector consisting of the op-amp comparator l8, and its summing network of resistor 22, capacitor 23, and diodes 24-25. This control circuit controls switch 32 thru resistors 36.

The control circuit for the --1 2 limit comprises resistors 41, 42, a phase sensitive demodulator consisting of field effect transistor 21 and its modulator reference source, and the detector consisting of the op-amp comparator l9 and its summing network of resistor 27, capacitor 28 and diodes 29, 30. This control circuit controls switches 34 through resistor 35. The gate of the phase sensitive demodulator reference signals which are square waves either in phase or 180 out of phase with the AC reference source.

A simple transistor inverter consisting of transistor 50, re: sistors 51, 48 and Zener diode 49, is used to generate STSZ. Table 1 shows the states of the switches for the three regions:

A l is equivalent to +15 volts. A is equivalent to l volts.

While it has been shown and described what is considered at and second servo motor feed circuits,

electronic angle limiting means comprising,

a first field effect transistor switch adapter to open and close said first feed circuit,

a first angle limiting feed circuit comprising first and second resistors connected in series between said second feed circuit and ground, and a second field effect transistor connected between the junction of said first and second resistor and said first feed circuit between said first switch and the servo motor, second angle limiting feed circuit comprising third and four resistors and a third field effect transistor switch connected in series between said second feed circuit and said first feed circuit between said first switch and the servo motor,

a first control circuit comprising a transistor inverter having its emitter and collector connected to a positive and a negative voltage source respectively and its collector connected to control said first switch,

a second control circuit comprising fifth and sixth resistors connected in series between said first and second servo motor feed circuits, a first phase sensitive demodulator and a first comparator means connected in series between the junction of said seventh and eighth resistors and the base of said transistor inverter, and means connecting the output of said second comparator means to control said third switch. 

1. In combination with a tangent servo system having first and second servo motor feed circuits, electronic angle limiting means comprising, a first field effect transistor switch adapter to open and close said first feed circuit, a first angle limiting feed circuit comprising first and second resistors connected in series between said second feed circuit and ground, and a second field effect transistor connected between the junction of said first and second resistor and said first feed circuit between said first switch and the servo motor, a second angle limiting feed circuit comprising third and four resistors and a third field effect transistor switch connected in series between said second feed circuit and said first feed circuit between said first switch and the servo motor, a first control circuit comprising a transistor inverter having its emitter and collector connected to a positive and a negative voltage source respectively and its collector connected to control said first switch, a second control circuit comprising fifth and sixth resistors connected in series between said first and second servo motor feed circuits, a first phase sensitive demodulator and a first comparator means connected in series between the junction of said seventh and eighth resistors and the base of said transistor inverter, and means connecting the output of said second comparator means to control said third switch. 